The process monitoring and control of the fabrication of VLSI circuits is critical for the achievement of high quality products. Traditionally, statistical process control (SPC) has been used as a means of identifying and solving fabrication problems. However, alternative methods are desirable because SPC is slow and expensive in that it requires of large amounts of empirical data gathering and analysis.
There are several commercially available instruments for measuring film thickness, each with limitations. For example, the ellipsometer which offers a noninvasive means for measuring film thickness is not useful for opaque films. The surface profilometer is useful for measuring opaque films, however, requires etching the film. The quartz crystal oscillator, which is capable of in-situ measurements of film thickness, only provides an indirect measurement of film thickness and furthermore cannot measure the film thickness at the point of interest, the wafer surface.